A centrifugal fan of this type as known for example from EP 1022469 B1 is capable of developing relatively high static pressures and has a good P/V characteristic, but cost and space considerations invariably demand a reduction in overall size and an increase in pressure and volumetric flow rate. In many applications, it is not possible to increase the fan speed in order to obtain these improvements; this applies for example to high-pressure cleaning apparatus in which one drive motor drives both a centrifugal fan and a high-pressure pump. High-pressure pumps are usually operated at speeds below 1500 rpm, forcing the centrifugal fan to operate at the same speed unless expensive and complex gearing is provided.
A high-pressure cleaning apparatus known from EP 0248282 B1 comprises such a pump and fan assembly with a drive shaft. To produce the required combustion air in an apparatus of this type, either a very high-volume centrifugal fan has to be used, or the speed has to be increased significantly, or a correspondingly smaller burner which requires less combustion air has to be installed.
Pump and fan assemblies with a drive shaft are also known from DE 3001571 A1 and DE 3115698 C1.